Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope (a PET tracer), which is introduced into the body on a biologically active molecule. Radiolabelled methyl iodide (11CH3I) and radiolabelled methane (11CH4) are used in the production of PET tracers. Automated chemistry modules designed to prepare such PET tracers, have been extremely valuable in clinical research and medicinal practices due to their ability to provide these PET tracers efficiently and quickly.
The production of 11CH3I, typically involves two steps. First, hydrogen is reacted with radiolabelled carbon dioxide (11CO2) over a nickel catalyst to produce 11CH4. Second, the 11CH4 is reacted with gaseous iodine to produce 11CH3I. The first step in the production of 11CH3I causes at least two problems.
First, water, oxygen, and nitrogen are byproducts and contaminants of the first step. These components are pollutants in the second step. Fortunately, water, oxygen, and nitrogen can be removed with common trap materials such as a supported sodium hydroxide material (for example, Ascarite™ sold by Aldrich Chemical Company) and/or phosphorus pentoxide (P2O5).
A second problem caused by the first step in the production of 11CH3I, i.e., the reaction of hydrogen with radiolabelled carbon dioxide, is more serious. Hydrogen, a necessary reactant in the first step, becomes a contaminant in the second step. Hydrogen competes with the desired reaction in the second step by reacting with gaseous iodine. A down stream product of hydrogen and iodine also damages downstream equipment, unless it is removed. Room temperature trap materials such as those employed to remove water, oxygen, and nitrogen are not available to separate hydrogen from the product stream in the automated production of 11CH3I.